The invention relates to improvements in apparatus for automatic, precise placement of objects, e.g., semiconductor wafers.
It is desirable to rapidly and automatically transfer semiconductor wafers from plastic storage boats to quartz boats (in which they are introduced into a diffusion furnace), and vice versa, without introducing particulate contamination (caused, for example, by scraping of wafers against the quartz boat during placement) onto the surfaces of the wafers. The quartz boats each include two upper, horizontal, rails that are spaced from each other. The two rails contain pairs of vertical slots used to retain the wafers. Scraping of a wafer can occur if the wafer is not properly aligned with the vertical slots during placement of the wafer into the quartz boat.
In a known system that is generally described in U.S. Pat. No. 4,493,606, a support member (or "paddle") transfers the wafers, one by one, from the plastic boat to the quartz boat. The paddle includes a center portion with vacuum openings and two outer, downwardly directed arms, which fit outside of upper rails of the quartz boat. The paddle includes a scanning system for identifying the locations of the quartz boat slots with respect to the paddle so that the wafer can be accurately aligned with a respective pair of slots during placement. A light source in each arm emits a beam of light that passes through a quartz boat slot (assuming that it is aligned therewith) is reflected by a respective mirror on the center portion of the paddle, and returns through the same slot to a sensing element located on the same arm. If the beam is not aligned with a slot, it is blocked by the rail. Prior to transfer of wafers, the unloaded quartz boat is moved past the paddle, and the positions of the slots are scanned and noted.
Certain quartz boats have rails configured such as not to permit light beams originating outside optic arms to pass through the slots.